This invention relates to the dehydrocyclization of acyclic hydrocarbons to provide aromatics employing as catalyst therefor a Group VIII metal-containing zeolite L which has been synthesized in a unique manner.
Catalytic reforming is a well known type of petroleum refinery operation which is employed to improve the octane rating of straight run gasolines. It involves converting naphtha fractions, i.e., paraffins which possess low octane ratings, to aromatics which possess high octane values. Hydrocarbon conversion reactions occurring during reforming include dehydrogenation of cyclohexanes to aromatics, dehydroisomerization of alkylcyclopentanes to aromatics, dehydrocyclization of acyclic hydrocarbons to aromatics, dealkylation of alkylbenzenes, isomerization of paraffins and hydrocracking reactions which product light gaseous hydrocarbons such as methane, ethane, propane and butanes.
Dehydrocyclization of acyclic hydrocarbons to provide aromatics is one of the principal reactions involved in reforming. Catalysts based on zeolite L are known to be more selective than other large pore zeolites for the dehydrocyclization reaction.
According to U.S. Pat. No. 4,435,283, paraffins undergo dehydrocyclization in the presence of hydrogen at a temperature of 430.degree.-550.degree. C. and in the presence of zeolite L which possesses cations of which at least 90% are alkali metal ions and at least one Group VIII metal. In the dehydrocyclization process described in U.S. Pat. No. 4,456,527, a hydrotreated, desulfurized hydrocarbon feed is contacted with zeolite L in the presence of a diluent, preferably hydrogen, at elevated pressure (1 atm to 500 psig) and temperature (450.degree.-550.degree. C.) and a liquid hourly space velocity (LHSV) of from about 0.1 to about 10 hr..sup.-1 Additional disclosures of the use of zeolite L in catalytic reforming/dehydrocyclization can be found in U.S. Pat. Nos. 4,517,306, 4,547,472 and 4,544,439 and UK patent application No. 2,116,450.
With the exception of U.S. Pat. No. 4,544,539 the foregoing prior art contemplates the use of zeolite L which has been prepared by the procedure described in U.S. Pat. No. 3,216,789.
Commonly assigned copending U.S. patent application Ser. No. 124,657 filed Nov. 24, 1987 describes a process for producing zeolite L free of zeolite T contaminant, the latter being a by-product when utilizing the zeolite L synthesis procedure of U.S. Pat. No. 3,216,789. Although the zeolite L produced by the process of U.S. patent application Ser. No. 124,657 is disclosed to be useful in dehydrogenating hydrocarbon compounds and converting paraffins to aromatics, no mention is made of employing the zeolite in association with a metal component much less a Group VIII metal component.
It is an object of this invention to provide a hydrocarbon conversion process and catalyst composition therefor which is selective for the dehydrocyclization of low octane paraffins to high octane aromatics.
It is a particular object of the invention to utilize a Group VIII metal-containing zeolite L catalyst composition for such dehydrocyclization, the zeolite L having been synthesized in the manner disclosed in aforesaid U.S. patent application Ser. No. 124,657.